Heat treatment of metals in a furnace requires an inert atmosphere, typically nitrogen. Reducing gases, such as carbon monoxide and hydrogen are added to the nitrogen to provide a buffer against oxygen leakage into the furnace.
The atmosphere compositions required to carry out the heat treating of ferrous and non-ferrous metals and alloys, the brazing of metals and the sintering of metal and ceramic powders are well known in the art.
Although in principle nitrogen is inert with respect to most metals and alloys at heat treating temperatures, in practice, reducing elements such as carbon monoxide and hydrogen (CO and H.sub.2) must often be added to the atmosphere composition in order to provide a buffer against inleak of oxygen into the furnace.
The oxygen that leaks into the furnace rapidly reacts with the CO and H.sub.2 present to form carbon dioxide and water (CO.sub.2 and H.sub.2 O) and as long as the CO/CO2 and H2/H2O ratios stay within desired limits the various heat treating processes can be carried out successfully. The actual CO/CO.sub.2 and H.sub.2 /H.sub.2 O ratios to be established will greatly depend on the particular process involved such as decarburization-free annealing, bright annealing, decarburization annealing and controlled oxide annealing of steels and these are well known in the art. For instance, for bright annealing of steels FIG. 1 shows an oxidation diagram for iron in CO/CO.sub.2 and H.sub.2 /H.sub.2 O mixtures. At 800.degree. C. atmospheres with CO/CO.sub.2 &gt;1.4 and H.sub.2 /H.sub.2 O&gt;1.8 will not oxidize steels (point B); in practice an atmosphere with CO/CO.sub.2 =5 and H.sub.2 /H.sub.2 O=6 (point A in FIG. 1) can advantageously be used since it will have an adequate buffer against O.sub.2 inleaks.
The known methods of preparing buffered atmospheres of this type fall in two main categories. The first category is generated atmospheres from endogenerators, exogenerators, ammonia dissociators. These atmospheres are inexpensive but they involve bulky equipment, are maintenance intensive and the atmospheres often lack consistency. The second category is atmospheres prepared from cryogenic nitrogen with the admixtures of hydrogen or methanol. These atmospheres are of high quality and very controllable but are also very expensive.
Several commercially practiced or proposed techniques to provide heat treating atmospheres for above-mentioned applications are known. One technique uses exothermic generators wherein atmosphere is produced in a refractory lined or a direct water-cooled combustion chamber with one or two burners to which a mixture of natural gas and air is delivered from controlled ratio pumping equipment. The generator is equipped with a cooler through which the products of combustion are discharged after removing a portion of the water produced in the reaction. There are two types of exothermic generators commonly used for ferrous annealing, the rich ratio exothermic generators in which the air to fuel ratio is typically about 6; the combustion atmosphere after cooling and removing most of the water will typically consist of 5% CO.sub.2, 11% CO, 14% H.sub.2 and 69% N.sub.2. Although the gas generated atmosphere has a low CO/CO.sub.2 ratio and is decarburizing, the atmosphere is suitable for oxide-free annealing of ferrous materials.
The other type is the purified exothermic generators in which the combustion gases are compressed and the CO.sub.2 and H.sub.2 O are removed by pressure-swing adsorption on molecular sieve beds. Atmosphere is suitable for decarb-free and oxide-free annealing of ferrous materials.
Another known technique uses endothermic generators diluted with nitrogen or exogas. In endothermic generators, the air to natural gas ratio is typically close to 25% of perfect combustion. Reaction takes place over a catalyst bed (usually Ni on Alumina brick) and external heat must be supplied to maintain the reaction. Gas composition from an endogenerator contains approximately 20% H.sub.2, 40% CO, balance N.sub.2. For annealing applications this gas is diluted in the furnace with N.sub.2 gas. The N.sub.2 can be from a cryogenic supply or impure N.sub.2 from membrane or PSA. Alternatively, the endogas can be diluted with exogas from an exogenerator.
Still another technique employs nitrogen/methanol systems wherein methanol is introduced directly into the furnace and at the furnace temperature dissociates into H.sub.2 and CO. For each gallon of methanol approximately 25 CF of CO and 50 CF of H.sub.2 are produced. N.sub.2 is also injected to obtain he desired atmosphere for annealing. The N.sub.2 can be from cryogenic supply or impure N.sub.2, from membrane or PSA.
A further known technique uses internally-mounted endothermic generators wherein the endothermic generator is mounted internally in the furnace thereby saving energy and eliminating the floor space requirement of an external generator. The internal generator is supplied with its own electrical heater and a precious metal catalyst is used for higher efficiency and lower space requirement. For annealing applications, dilution of the endogas with N.sub.2 can be used. The N.sub.2 can be from a cryogenic supply or impure N.sub.2 from membrane or PSA.
A still further technique is one in which endothermic conversion of impure nitrogen is used. In this process an endogenerator type reactor is used to convert the O.sub.2 present in nitrogen generated by membrane to H.sub.2 and CO. Typical membrane purity is low (between 3 and 5%). Resulting atmospheres have between 5 to 8% CO and 10 to 16% H.sub.2. Since only a small amount of heat is generated at these low O.sub.2 concentrations, it is necessary to preheat the reactants.
Finally, another technique employs the "in-situ" conversion of impure nitrogen. Various methods have been suggested of premixing nitrogen obtained from membranes or PSA with a predetermined quantity of hydrogen and/or hydrocarbon and injecting this mixture into the hot zone of the furnace. The amount of hydrogen and/or hydrocarbon used is several times the amount required for conversion of the oxygen in the impure nitrogen to the complete oxidation products CO.sub.2 and H.sub.2 O. Location and method of injection can be critical.
The aforesaid known techniques all have drawbacks such that they are not totally satisfactory heat treating atmospheres. Exothermic generators are separate pieces of equipment that need to be maintained. Cooling of the combustion gases and subsequent reheating involves thermal inefficiencies. Rich ratio exothermic generators with or without refrigerant dryers are relatively simple to operate and capital costs are modest. However resultant atmospheres are not of high quality and are not suitable for decarb-free annealing. Purified exogenerator atmospheres are of high quality, however capital and operating costs are high, since it involves compressing the combustion gases and there are losses in the use of molecular sieve beds.
Diluted endothermic gas gives a high quality atmosphere; endothermic generators are however more costly to operate than exogenerators and again involve a separate piece of equipment which must be controlled and maintained. Thermal inefficiency due to atmosphere reheating is also a disadvantage.
Nitrogen/Methanol delivers high quality atmosphere with low capital and maintenance costs. However operating costs are high due to the high cost of methanol. Thermal efficiency is also low since the furnace must provide the heat to dissociate the methanol and bring the injected gases to the furnace temperature.
Internally mounted endothermic generators are relatively new in the technology. Their principal advantage is that no separate generator is required. Furnace atmosphere controls are used to control the output of the generator avoiding duplication. The heat of reaction is not lost so thermal efficiency is high. Standard Nickel or precious metal reforming catalyst is used as in stand-alone generators. Since reforming reactions are slow, space velocities are low and this makes the system bulky which is a disadvantage for internally mounted systems. For example, for one commercially available system, the internally mounted generator delivering 800 SCFH of endogas measures 10.5" diameter and is 32" long.
An example of such a generator is described in U.S. Pat. No. 5,160,380 issued Nov. 3, 1992 to Vocke et al. entitled PROCESS FOR IMPROVED PREPARATION OF TREATMENT GAS IN HEAT TREATMENTS.
The endothermic conversion of the oxygen in membrane nitrogen to CO and H.sub.2 has all the disadvantages of external endogenerators and substantially more heat must be provided than for the air/natural gas case. Thermal efficiency is low and capital cost is high.
"In-situ" conversion of impure nitrogen without the use of a catalyst. The principal disadvantage of these methods is that the oxygen in the impure nitrogen will initially give rise to the total oxidation products H.sub.2 O and CO.sub.2. If only H.sub.2 is used, sufficient H.sub.2 must be supplied to give the desired H.sub.2 /H.sub.2 O and CO.sub.2. The need for an external H.sub.2 source makes this approach expensive. If hydrocarbons such as methane or propane are used, the desired CO/CO.sub.2 and H.sub.2 /H.sub.2 O ratios are obtained through reforming of CO.sub.2 and H.sub.2 O in the furnace by adding sufficient excess hydrocarbon. These reforming reactions are slow at typical heat treating temperatures particularly when using methane. An example of this is shown in FIG. 2. The desired atmosphere can only be obtained if furnace temperatures are high enough and the gas residence time is long enough for sufficient reforming to take place. Gas composition will therefore be dependent on the operation of the furnace.
Other background references relating to the present subject matter are as follows.
U.S. Pat. No. 5,298,090 issued Mar. 29, 1994, to Garg et al. entitled "ATMOSPHERES FOR HEAT TREATING NON-FERROUS METALS AND ALLOYS" discloses a process for producing low-cost atmospheres suitable for annealing, brazing, and sintering non-ferrous metals and alloys from non-cryogenically produced nitrogen containing up to 5% residual oxygen. According to the process, suitable atmospheres are produced by 1) pre-heating the non-cryogenically produced nitrogen stream containing residual oxygen to a desired temperature, 2) mixing it with more than a stoichiometric amount a hydrocarbon gas, 3) passing it through a reactor packed with a platinum group of metal catalyst to reduce the residual oxygen to very low levels and convert it to a mixture of moisture and carbon dioxide, and 4) using the reactor effluent stream for annealing, brazing, and sintering non-ferrous metals and alloys in a furnace. The key features of the disclosed process include 1) pre-heating the non-cryogenically produced nitrogen containing residual oxygen to a certain minimum temperature, 2) adding more than a stoichiometric amount of a hydrocarbon gas to the pre-heated nitrogen stream, and 3) using a platinum group of metal catalyst to initiate and sustain the reaction between oxygen and the hydrocarbon gas.
U.S. Pat. No. 5,259,893, issued Nov. 9, 1993 to Bonner et al., entitled "IN-SITU GENERATION OF HEAT TREATING ATMOSPHERES USING A MIXTURE OF NON-CRYOGENICALLY PRODUCED NITROGEN AND A HYDROCARBON GAS", discloses a process for generating in-situ low-cost atmospheres suitable of annealing and heat treating ferrous and non-ferrous metals and alloys, brazing metals, sealing glass to metals, and sintering metal and ceramic powders in a continuous furnace from non-cryogenically produced nitrogen containing up to 5% residual oxygen. The disclosed process involves mixing nitrogen gas containing residual oxygen with a predetermined amount of a hydrocarbon gas, feeding the gaseous mixture through a nonconventional device into the hot zone of a continuous heat treating furnace, converting residual oxygen to an acceptable form such as a mixture of moisture and carbon dioxide, a mixture of moisture, hydrogen, carbon monoxide, and carbon dioxide, or a mixture of carbon monoxide, moisture, and hydrogen, and using the resultant gaseous mixture for annealing and heat treating metals and alloys, brazing metals, sintering metal and ceramic powders, and sealing glass to metals.
U.S. Pat. No. 5,254,180 issued Oct. 19, 1993 to Bonner et al., entitled "ANNEALING OF CARBON STEELS IN A PRE-HEATED MIXED AMBIENTS OF NITROGEN, OXYGEN, MOISTURE AND REDUCING GAS", discloses an improved process for producing high moisture containing nitrogen-based atmospheres suitable for oxide and decarburize annealing of carbon steels from non cryogenically generated nitrogen. These nitrogen-based atmospheres are produced by mixing non-cryogenically generated nitrogen containing less than 5.0 vol. % residual oxygen with a specified amount of hydrogen, humidifying the gaseous feed mixture, feeding the gaseous mixture into the heating zone of a furnace through a diffuser, and converting in-situ the residual oxygen present in it to moisture. According to the present invention, the total amount of hydrogen required for producing suitable atmospheres can be minimized by simultaneously humidifying the feed gas and controlling the residual oxygen level in it. The key features of the present invention include a) humidifying the feed gas prior to introducing it into the heating zone of a furnace operated above about 600.degree. C., b) selecting the level of residual oxygen in the feed gas in such a way that it minimizes hydrogen consumption, and c) using enough amount of hydrogen to convert completely the residual oxygen present in the feed gas to moisture and to maintain pH.sub.2 /pH.sub.2 O ratio in the heating zone of the furnace below about 2 for oxide annealing and at least 2 for decarburize annealing carbon steels.
U.S. Pat. No. 5,242,509, issued Sep. 7, 1993 to Rancon et al. entitled "PROCESS OF THE PRODUCTION OF AN ATMOSPHERE FOR THE THERMAL TREATMENT OF METALS AND THERMAL TREATMENT APPARATUS", describes a process wherein the thermal treatment atmosphere is obtained by catalytic reaction of an impure mixture of nitrogen, advantageously obtained by permeation or adsorption, and hydrocarbon, the catalytic reaction being carried out at a temperature between 400.degree. and 900.degree. C., typically between 500.degree. and 800.degree. C., with a noble metal base catalyst, typically platinum or palladium on alumina support. The reaction may be carried out in a reactor placed inside or outside the furnace.
U.S. Pat. No. 5,221,369 issued Jun. 22, 1993 to Bowe et al., entitled "IN-SITU GENERATION OF HEAT TREATING ATMOSPHERES USING NON-CRYOGENICALLY PRODUCED NITROGEN", discloses a process for generating in-situ low-cost atmospheres suitable for annealing and heat treating ferrous and non-ferrous metals and alloys, brazing metals and ceramics, sealing glass to metals, and sintering metal and ceramic powders in a continuous furnace from non-cryogenically produced nitrogen containing up to 5% residual oxygen. The disclosed process involves mixing nitrogen gas containing residual oxygen with a pre-determined amount of a reducing gas such as hydrogen, a hydrocarbon, or a mixture thereof, feeding the gaseous mixture through a non-conventional device into the hot zone of a continuous heat treating furnace, converting residual oxygen to an acceptable form such as moisture, a mixture of moisture and carbon dioxide, or a mixture of moisture, hydrogen, carbon monoxide and carbon dioxide, and using the resultant gaseous mixture for annealing and heat treating metals and alloys, brazing metals and ceramics, sintering metal and ceramic powders, and sealing glass to metals.
U.S. Pat. No. 5,069,728 issued Dec. 3, 1991 to Rancon et al., entitled "PROCESS FOR HEAT TREATING METALS IN A CONTINUOUS OVEN UNDER CONTROLLED ATMOSPHERE", describes the heat treating of metals by continuous longitudinal passage of metallic pieces in an elongated treating zone under controlled atmosphere having a high temperature upstream end where the controlled atmosphere comprises nitrogen and reducing chemical substances, such as hydrogen, possibly carbon monoxide, and a down-stream cooling end under an atmosphere essentially formed by introducing nitrogen. In the high temperature upstream end, the nitrogen which constitutes the atmosphere is supplied by introducing nitrogen with a residual oxygen content not exceeding 5%. The nitrogen introduced in the downstream cooling end is substantially free of oxygen. Application of the process to the annealing of metallic pieces.
U.S. Pat. No. 5,057,164 issued Oct. 15, 1991 to Nilsson et al., entitled "PROCESS FOR THERMAL TREATMENT OF METALS", discloses a process for thermal treatment of metals by passage of metallic pieces into an elongated zone under a controlled atmosphere, having an upstream section at an elevated temperature, where the controlled atmosphere comprises nitrogen and reductive chemicals, particularly hydrogen, possibly carbon monoxide; and a downstream section at a lower temperature under a controlled atmosphere. The invention is characterized by the fact that in the upstream section at an elevated temperature, the atmosphere comprises nitrogen having a residual content of oxygen between 0.5% and 5% produced by separation of air using permeation or adsorption techniques. The reductive chemicals are present at all times in a content at least sufficient to eliminate the oxygen admitted with the nitrogen. The controlled atmosphere in the section downstream from the elongated thermal treatment zone is formed by admission of a gaseous flow taken from the upstream section at an elevated temperature and transferred directly into the downstream section at a lower temperature.
Australian Patent Application 34059/93 dated Sep. 16, 1993 to Frey, entitled "METHOD AND APPARATUS FOR FORMING A HEAT TREATING ATMOSPHERE", describes a method of forming a heat treating atmosphere by removing at least a substantial portion of the oxygen contained within a feed stream of air to produce a nitrogen rich gas and an oxygen enriched waste gas, mixing the nitrogen rich gas and a substituted or unsubstituted hydrocarbon gas to form a first mixture; and reacting the first mixture in the present of a non-noble metal catalyst to form said heat treating atmosphere containing a predominant amount of nitrogen gas and no more than trace amounts of carbon dioxide and water vapor.
PCT Patent WO 93/21350 Gross et al. dated Oct. 28, 1993 and entitled "METHOD OF PRODUCING A PROTECTIVE OR REACTIVE GAS FOR THE HEAT TREATMENT OF METALS" discloses nitrogen produced by non-cryogenic methods, such as those using pressure-change adsorption or membrane installations, cannot owing to its high oxygen content of about 0.1 to 5% V/V, be used for the heat treatment of metals, or can only be used to a limited degree. The invention proposes an endothermic catalytic conversion of the oxygen contained in the nitrogen by means of hydrocarbons to give a protective gas which is suitable for the heat treatment of metals.